In a mass spectrometer, an ion optical element called the “ion guide” is used for focusing ions coming from the previous stage, accelerating them in some cases, and sending them into a mass analyzer, such as a quadrupole mass filter. An ion guide generally has a multi-pole structure with four or eight cylindrical (or tubular) rod electrodes arranged parallel to each other around an ion beam axis. Normally, in the quadrupole or octapole ion guide, the same radio-frequency (RF) voltage is applied to one pair of rod electrodes facing each other across the ion beam axis, while another RF voltage, which is identical in amplitude and opposite in phase to the aforementioned RF voltage, is applied to another pair of rod electrodes neighboring the aforementioned pair in the circumferential direction. The thus applied RF voltages create an RF electric field in the space surrounded by the rod electrodes, and the ions are transported to the subsequent stage while being oscillated in this RF electric field.
In an ion guide disclosed in Patent Document 1, virtual rod electrodes, each of which consists of a plurality of plate electrodes arrayed along the ion beam axis, are used in place of the rod electrodes. In the virtual-rod configuration, a direct-current (DC) electric field having a potential gradient along the ion beam axis can be created so as to accelerate, or conversely, decelerate ions while exploiting the advantage of high ion-focusing performance of the multipole ion guide.
As already explained, ion guides are primarily used to transport various ions produced by an ion source to a mass analyzer. However, the particles introduced into the ion guide normally contain not only ions originating from a sample, but also neutral particles, such as the sample molecules which have not been ionized in the ion source. Such neutral particles, if allowed to reach the mass analyzer, will cause a measurement noise. Furthermore, they will also contaminate the mass analyzer. Given these problems, a curved ion guide using curved rod electrodes has been conventionally used to remove neutral particles in the course of their travel through the ion guide (for example, refer to Patent Document 2 or 3).
FIG. 8 is a schematic perspective view of one example of the curved ion guide. As shown, this ion guide 2 has four curved rod electrodes 201, 202, 203 and 204. Due to the effect of the RF electric field, ions which have originated from a sample follow a curved path along the shape of the ion guide, whereas neutral particles, which have no electric charges and will not be affected by the RF electric field, travel straightly through the ion guide 2, to be eventually eliminated by being discharged from the ion guide 2 or coming in contact with the curved rod electrodes 201-204.
Since the ions introduced into the ion guide 2 have certain amounts of energy, it is actually difficult to achieve both the focusing and curving of the ions along the curved path by using only the RF electric field. To address this problem, a curved ion guide disclosed in Patent Document 3 not only employs the curved shape of the rod electrodes but also applies a deflecting DC voltage to the curved rod electrodes or auxiliary electrodes provided independently of the curved rod electrodes, so as to create, in the space surrounded by the curved rod electrodes, a DC electric field which acts on the ions and curves them toward the inside of the curved path (as indicated by the arrow R in FIG. 8).
FIGS. 9 and 10 are configuration diagrams of the curved rod electrodes and the auxiliary electrodes described in Patent Document 3 as well as the circuit blocks for applying voltages to those electrodes. The system shown in FIG. 9 has no auxiliary electrodes. The thick white arrow in this figure points toward the inside of the curved path in the curved ion guide 2 (i.e. inward along the radial direction of the curved central axis, which is a segment of an arc). The voltage sources 501-504 apply an RF voltage VRF to the two curved rod electrodes 202 and 204 facing each other among the four curved rod electrodes 201-204, as well as an RF voltage −VRF with the same amplitude and opposite polarity to the other two curved rod electrodes 201 and 203. As a result, an RF electric field for focusing ions while oscillating them in the previously described manner is created in the space surrounded by the curved rod electrodes 201-204. The voltage sources 501-504 also apply a DC voltage −VDEF whose polarity is opposite to that of an ion to be analyzed (which is a positive ion in the present example) to the two curved rod electrodes 201 and 202 located on the inside of the curved path, as well as a DC voltage VDEF having the same polarity as that of the ion to be analyzed to the two curved rod electrodes 203 and 204 located on the outside of the curved path. As a result, a DC electric field for attracting ions toward the inside of the curved path, i.e. in the direction indicated by the thick white arrow in the figure, is created in the space surrounded by the curved rod electrodes 201-204.
The system shown in FIG. 10 has auxiliary electrodes 205 and 206. The voltage source 511 and 512 apply an RF voltage VRF to the two curved rod electrodes 202 and 204 facing each other among the four curved rod electrodes 201-204, as well as an RF voltage −VRF with the same amplitude and opposite polarity to the other two curved rod electrodes 201 and 203. The voltage source 514 applies a DC voltage −VDEF whose polarity is opposite to that of an ion to be analyzed to the auxiliary electrode 205 located on the inside of the curved path. The voltage source 513 applies a DC voltage VDEF having the same polarity as that of the ion to be analyzed to the auxiliary electrode 206 located on the outside of the curved path. As a result, similar to the system of FIG. 9, a DC electric field for attracting ions toward the inside of the curved path is created, in the form of being superposed on the ion-focusing RF electric field, in the space surrounded by the curved rod electrodes 201-204.
By applying appropriate deflecting DC voltages to either the curved rod electrodes or the auxiliary electrodes in the previously described manner, it is possible to curve ions along the curved path of the ion guide 2 and guide them to the exit end so as to improve the ion transmission efficiency. However, such conventional systems have the following problem.
That is to say, the DC electric field which acts on the ions in the radial direction within the inner space of the ion guide 2 in the previously described manner functions as an energy filter which allows the passage of ions only within a specific range of kinetic energy. Accordingly, the transmission efficiency of the ions deteriorates if the variation in the kinetic energy the ions introduced into the ion guide 2 is relatively large. To avoid this situation, it is necessary to reduce the relative variation of energy by comparatively increasing the kinetic energy of the ions introduced into the ion guide 2. For the ion guide disclosed in Patent Document 3, a difference in the ion transmission efficiency depending on the presence or absence of the deflecting DC electric field has been investigated for an ion having a considerably high kinetic energy of 100 eV. However, a study by the present inventor has revealed that, when ions with such a high kinetic energy are introduced into a curved ion guide, it is difficult to adequately focus the ions by using only the RF electric field. This constitutes a cause of deterioration in the ion transmission efficiency.